1. Field of the Invention
The present invention relates to laser beam generating apparatus and method, and more particularly to laser beam generating apparatus and method for generating CW (continuous wave) or pulse laser beams of short wavelength by using a non-linear optical crystal device (BBO).
2. Description of the Related Art
Recently, the microstructure of semiconductor devices has been increasingly enhanced, and this enhancement of the microstructure of the semiconductor devices has also caused increasing requirements to shorten the wavelengths of light beams emitted from light sources in an exposure process for use in a semiconductor device manufacturing process. Of these light sources used in the exposure process, a YAG full solid-state laser for generating fifth higher harmonic wave of 213 nm in wavelength by using .beta.-BaB.sub.2 O.sub.4 (hereinafter referred to as "BBO") which is a non-linear optical crystal device has been expected as a light source for the semiconductor exposure process.
Besides, the market has required in an earlier stage that light sources of the next generation should have wavelengths of 200 nm or less. At the present situation, the requirement to shorten the wavelength of the light source to be less than 200 nm has not been satisfied for the full solid-state laser insofar as the BBO crystal device is used because the transmittance of the BBO crystal is sharply reduced in the wavelength range of 200 nm or less. For example, in the case of 3.5 mm-thickness BBO crystal, the transmittance at the same wavelength as an ArF excimer laser is sharply reduced from 70% to 10% which contains the surface reflectance of the end face of the crystal.
In order to avoid this problem, there has been proposed a method of producing 205 nm or 201 nm wavelength. However, this method has not yet been practically used (Japanese Unexamined Patent Applications No. Sho-62-162374, No. Sho-62-162377). Further, there has been proposed a method of producing 193 nm wavelength from the sum of 213 nm wavelength of YAG fifth higher harmonic and 2.0 .mu.m of infrared ray by using an LBO (LiB.sub.3 O.sub.5) crystal device at the final stage. However, a light source based on this method outputs only small output power, and it is difficult to obtain large output power.
The BBO crystal is one of crystals which are most suitable for practical use and highest reliability in lots of wavelength-converting non-linear optical crystals. If the full solid-state laser having the light source which is easier in maintenance, more compact in size and higher in efficiency and quality as compared with the ArF excimer laser, can produce the same wavelength (193 nm) as the ArF excimer laser with the same-level laser power as the ArF laser, its contribution to the industries would be extremely great.